1. Field of the Invention
The invention generally relates to DC (Direct Current) motors used in various applications, such as hard disk drive motors, cooling fans, drive motors for appliances, etc.
2. Description of the Related Art
Electric motors are used to produce mechanical energy from electrical energy, used in a number of applications, including different household appliances, pumps, cooling fans, etc. Electric motors are generally classified as either alternating current (AC) motors or direct current (DC) motors.
Motors generally include a rotor, which is the non-stationary (moving) part of the motor, and a stator, which is the stationary part of the motor. The stator generally operates as a field magnet (e.g., electromagnet), interacting with an armature to induce motion in the rotor. The wires and magnetic field of the motor (typically in the stator) are arranged so that a torque is developed about the rotor's axis, causing rotation of the rotor. A motor typically also includes a commutator, which is an electrical switch that periodically reverses the current direction in the electric motor, helping to induce motion in the rotor. The armature carries current in the motor and is generally oriented normal to the magnetic field and the torque being generated. The purpose of the armature is to carry current crossing the magnetic field, thus creating shaft torque in the motor and to generate an electromotive force (EMF).
In a typical brushed DC motor, the rotor comprises one or more coils of wire wound around a shaft. Brushes are used to make mechanical contact with a set of electrical contacts (called the commutator) on the rotor, forming an electrical circuit between the DC electrical source and the armature coil-windings. As the armature rotates on an axis, the stationary brushes come into contact with different sections of the rotating commutator. The commutator and brush system form a set of electrical switches, each firing in sequence, such that electrical-power always flows through the armature coil closest to the stationary stator (permanent magnet). Thus an electrical power source is connected to the rotor coil, causing current to flow and producing electromagnetism. Brushes are used to press against the commutator on the rotor and provide current to the rotating shaft. The commutator causes the current in the coils to be switched as the rotor turns, keeping the magnetic poles of the rotor from ever fully aligning with the magnetic poles of the stator field, hence maintaining the rotation of the rotor. The use of brushes creates friction in the motor and leads to maintenance issues and reduced efficiency.
In a brushless DC motor, the commutator/brush-gear-assembly (which is effectively a mechanical “rotating switch”) is replaced by an external electronic switch that's synchronized to the rotor's position. Brushless DC motors thus have an electronically controlled commutation system, instead of a mechanical commutation system based on brushes. In a brushless DC motor, the electromagnets do not move, but rather the permanent magnets rotate and the armature remains static. This avoids the problem of having to transfer current to the moving armature. Brushless DC motors offer a number of advantages over DC motors featuring brushes, including higher efficiency and reliability, reduced noise, longer lifetime (no brush erosion), the elimination of ionizing sparks from the commutator, and overall reduction of electromagnetic interference (EMI).
One issue oftentimes taken into consideration when designing motors, more specifically brushless motors, is the power required to operate the motor. One technique to reduce power in some applications has been the introduction of Three Phase Brushless DC (TPDC) Motors. Such motors are used in a variety of applications, for example in driving cooling fans. However, in certain cooling applications the desired power is not always available, oftentimes requiring the use of a non-ideal regulator to reduce the available voltage level to a level useable by the fan being driven by the motor. For example, a regulated voltage in the 5V DC to 12 V DC range may need to be derived from a 9V DC to 21V DC voltage provided by a battery in portable applications, or from a 20V DC to 48V DC voltage available in industrial applications. Because regulators are typically not 100% efficient, the efficiencies of all regulators between the source and point of use have to be multiplied to obtain an overall efficiency. Efficient switching step-down regulators, commonly known as “buck” regulators are widely used, and typically operate at 95% efficiency. If two regulators of this type are used in series, overall efficiency decreases to 90%.
Most current solutions are limited to developing very efficient buck regulators capable of high input voltages at relatively low current, and producing low voltage, high current outputs. Buck regulators can regulate current through an inductive element as a means of regulating output voltage. One method of implementing this control method is to use a fixed frequency PWM signal, and vary duty cycle based on load to maintain constant current. While such a control method can result in a very efficient voltage regulator, the efficiency of the regulator will have to be multiplied with the efficiency of the motor driver driving the fan to determine the overall electrical efficiency of the entire cooling subsystem. Anything less than 100% efficiency results in energy being lost in the conversion from one rail voltage to another. Further losses are encountered during the commutation process, as there are finite losses in the switching transistors that may be used in commutation. For example, a very efficient buck regulator—running at 95% efficiency—driving a motor driver that is 90% efficient would yield an overall electrical efficiency of 85.5%. In many cases this would represent less than the desired efficiency.
Other corresponding issues related to the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.